In recent years, along with the Internet and E-mails becoming widespread and communication networks becoming upgraded, occasions for accessing various information from locations other than fixed locations such as an office and a home are increasing. As a result, portable electronic devices such as a cellular telephone device, a PHS (Personal Handyphone System) or a PDA (Personal Digital Assistants) are coming into wider use. Moreover, similarly to ordinary computers, the demand for colorfully displaying various information is increasing for these portable electronic devices, and the use of a color liquid crystal display is rapidly spreading.
Incidentally, as a display for such portable electronic devices, the use of an EL (Electroluminescence) display device has been attracting attention, together with a conventional liquid crystal display (LCD). The EL display device uses a substance that has a characteristic of emitting light when an electrical voltage is applied thereto, and one including an organic substance such as carbon is referred to as organic EL (hereinafter also referred to as OLED (Organic Light-Emitted Diode)).
The organic EL display device is a self-luminous display similar to a CRT (Cathode Ray Tube) and a plasma display, and has various characteristics (features) such as high brightness, high resolution, high contrast, a wide viewing angle, an ultrathin structure, and low power consumption. Accordingly, attention has been focused on its use for a portable electronic device that is small in size with the electric power consumption thereof restricted. There is a type of organic EL element in which a single element emits a single color such as green, blue and red, and a multicolor display can be configured by combining these elements. In addition, since the display response speed thereof is very high and about 1,000 times higher than that of a liquid crystal display, smooth animated images can be displayed. Furthermore, in addition to its capability of expressing tones of colors, a backlight is not required, thus a thin structure is achieved, and it is possible to further suppress power consumption depending on the usage.
However, the EL display device having such superior characteristics has a problem in that the brightness thereof decreases in proportion to the period of time for displaying (emitting light). In the EL display device, light emission of each individual EL display element is controlled, thereby configuring a color image and the like as a whole. Accordingly, in a case such as, for example, displaying a menu screen, when the same point on the display is always turned on (emitting light), the EL display element of that portion is deteriorated faster than the EL display elements of other portions. When deterioration occurs, the brightness of the portion decreases (burn-in). Therefore, a difference of a degree of deterioration arises among the EL display elements, and a definite difference among portions with high brightness and portions with low brightness appears on the display.
In the portable electronic devices as represented by a cellular telephone device, there are relatively more application purposes for displaying the same static image for a long time, such as a case of reading text information, than a case of watching an animated image. Therefore, there is a high risk of assuming a usage pattern that causes nonuniform displaying, instead of a usage pattern of equally using the entire display. Accordingly, the OLED has a problem in that, when the display aspects of the pixels configuring the display vary due to deterioration (burn-in and the like) of the display elements, this variation appears as nonuniform brightness, thereby decreasing the quality of displaying images. Moreover, in the LCD as well, burn-in of a liquid crystal would occur due to displaying for a long time, and thus there is a possibility that a problem of similar display quality deterioration occurs.
Accordingly, various terminals have been proposed in which countermeasures against the aforementioned burn-in phenomenon are taken (see Patent Documents 1 to 4, for example).
Patent Document 1 discloses a radio communication terminal in which a display speed for displaying data is set, and an image is displayed at the set speed. Patent Document 2 discloses a display unit that switches reversing display and non-reversing display when detecting a connection to an external power supply. Patent Document 3 discloses a terminal device in which a display area is divided into a plurality of display areas, which are changed depending on time. Patent Document 4 discloses a cellular telephone device in which the chromaticity coordinates of white display of an image are determined by calculation, such that the RGB rate configuring the white display portion of an image is smaller for a color corresponding to an element of which aging degradation speed for luminescence brightness is higher among each RGB light emitting element.
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2002-268601
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2003-186434
Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2003-223160
Patent Document 4: Japanese Unexamined Patent Application, First Publication No. 2003-280588